xdg-realpath doesn't behave as documented for error cases

After rereading the xdg-realpth code I noticed that apparently some commits got lost regarding the Makefile (thanks for !119) and error handling.

In case of a path not resolving to a file gets passed:

• whether one gets an error message explaining what happened depends on being past the double hyphen --.
• It exits with the exit code 1 rather than the correct exit code 2 (as documented)
• If multiple files are passed, it aborts immediately instead of continuing like one would expect from a realpath or readlink -f implementation. This behavior also isn't properly documented.